Method and apparatus for space division multiple access receiver

ABSTRACT

Methods and systems consistent with this invention receive a plurality of transmitted signals in a receiver having a plurality of receive elements, wherein each transmitted signal has a different spatial location. Such methods and systems receive the plurality of transmitted signals at the plurality of receive elements to form a plurality of receive element signals, form a combined signal derived from the plurality of receive element signals, and detect each of the plurality of transmitted signals from the combined signal by its different spatial location. To achieve this, methods and systems consistent with this invention generate a plurality of arbitrary phase modulation signals, and phase modulate each of the plurality of receive element signals with a different one of the phase modulation signals to form a plurality of phase modulated signals. Such methods and systems then combine the plurality of phase modulated signals into a combined signal, generate expected signals, and cross-correlate the combined signal with the expected signals to form correlation signals. Such methods and systems then store the correlation signals in a correlation signal memory and analyze the correlation signals to extract information from the transmitted signals.

RELATED APPLICATION

This application is a continuation of U.S. Pat. No. 7,496,129 filed onJul. 31, 2007, which is a continuation of U.S. Pat. No. 7,251,286, filedon Nov. 9, 2004, which is a continuation of U.S. Pat. No. 6,823,021,filed on Oct. 27, 2009, all of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to wireless communication networks, andmore particularly to space-division multiple access (SDMA) in wirelesscommunication networks.

BACKGROUND OF THE INVENTION

Wireless communication services are an increasingly common form ofcommunication, and demand for wireless services continues to grow.Examples of wireless services include cellular mobile telephones,wireless Internet service, wireless local area computer networks,satellite communication networks, satellite television, and multi-userpaging systems. Unfortunately, these communication systems are confinedto a limited frequency spectrum either by practical considerations or,as is often the case, by government regulation. As the maximum number ofusers, or “capacity,” of these systems is reached, user demand for moreservice may be met by either (1) allocating more frequency spectrum tothe wireless service, or (2) using the allocated frequency spectrum moreefficiently. Because the frequency spectrum is limited and cannot keeppace with user demand, there is a critical need for new technology thatuses the allocated frequency spectrum more efficiently.

Wireless communication systems are generally composed of one or morebase stations through which wireless users, such as mobile telephoneusers, gain access to a communications network, such as a telephonenetwork. A base station serves a number of wireless users, fixed ormobile, in a local area. To increase the capacity of the systems,service providers may install more base stations, reducing the area andthe number of users handled by each base station. This approachincreases system capacity without allocating more spectrum frequencybands, but is quite costly because it requires significantly morehardware.

Another approach to using the frequency spectrum more efficiently is byimproving “multiple access” techniques. Multiple access techniques allowmultiple users to share the allocated frequency spectrum so that they donot interfere with each other. The most common multiple access schemesare Frequency-Division Multiple Access (FDMA), Time-Division MultipleAccess (TDMA), Code-Division Multiple Access (CDMA), and more recentlySpace-Division Multiple Access (SDMA).

FDMA slices the allocated frequency band into multiple frequencychannels. Each user transmits and receives signals on a differentfrequency channel to avoid interfering with the other users. When oneuser no longer requires the frequency channel assigned to it, thefrequency channel is reassigned to another user.

With TDMA, users may share a common frequency channel, but each useruses the common frequency channel at a different time. In other words,each user is allocated a time slot in which the user may transmit andreceive. Thus, TDMA interleaves multiple users in the available timeslots.

CDMA allows multiple users to share a common frequency channel by usingcoded modulation schemes. CDMA assigns distinct codes to each of themultiple users. The user modulates its digital signal by a widebandcoded pulse train based on its district code, and transmits themodulated coded signal. The base station detects the user's transmissionby recognizing the modulated code.

In SDMA, a system may separate a desired user's signal from othersignals by its direction of arrival, or spatial characteristics. This issometimes referred to as “spatial filtering.” Thus, even though twousers may be transmitting on the same frequency at the same time, thebase station may distinguish them because the transmitted signals fromthe users are arriving from different directions. It is possible to useSDMA in combination with FDMA, TDMA, or CDMA.

A conventional SDMA receiver requires an array of multiple receiveelements. Further, a conventional SDMA receiver uses a bank of phaseshifters that cooperates with the receive element array to form a “beam”that “looks” in a particular direction. It is generally more desirableto form multiple beams, each directed toward a different direction,i.e., toward different users. The more beams, the more simultaneoususers the SDMA system may handle operating on the same frequency at thesame time. The more beams, however, the more complicated the SDMAreceiver. For instance, each beam may require a separate bank of phaseshifters and circuits that perform signal tracking. Additionally, eachbeam may require a separate “signal combiner,” which combines thesignals received from each receive element to form a “combined signal.”Further still, each combined signal may require a separate signaldetector, which detects the transmitted signal from the user. Thishardware complexity greatly increases the cost of an SDMA receiver.

Using well known algorithms, hardware complexity may be reduced byperforming phase shifting, signal tracking, signal combining and signaldetecting in signal processing software. Current signal processingtechniques, however, have difficulty identifying and tracking largenumbers of simultaneously transmitted signals on the same frequency,particularly in a “multipathing” environment. A multipathing environmentis one where transmitted signals may reach the receiver over multiplepaths. For instance, a transmitted signal may reach the receiver (1)directly, and (2) indirectly after reflecting off objects. Multipathsignals may also further complicate the complexity of the conventionalSDMA receiver in the same manner as described above.

Thus, there is a need to provide an improved SDMA receiver that cansimultaneously receive from multiple directions and operate in amultipath environment without likewise increasing hardware or softwarecomplexity of the receiver.

SUMMARY OF THE INVENTION

The summary and the following detailed description should not restrictthe scope of the claimed invention. Both provide examples andexplanations to enable others to practice the invention.

Methods and systems consistent with this invention may incorporate amulti-element receive signal array that may achieve polarizationindependent isotropic reception, with power gain that may be greaterthan isotropic. Such methods and systems may receive multiple signalshaving the same or different carrier frequencies, distinguish thesignals, and establish their directions of arrival.

Methods and systems consistent with this invention receive a pluralityof transmitted signals in a receiver having a plurality of receiveelements, wherein each transmitted signal has a different spatiallocation. Such methods and systems receive the plurality of transmittedsignals in the plurality of receive elements to form a plurality ofreceive element signals, form a combined signal derived from theplurality of receive element signals, and detect the plurality oftransmitted signals from the combined signal by its different spatiallocation.

To achieve this, methods and systems consistent with this inventiongenerate a plurality of phase modulation signals that may be arbitraryor uncorrelated, and phase modulate each of the plurality of receiveelement signals with a different one of the phase modulation signals toform a plurality of phase modulated signals. Such methods and systemsthen combine the plurality of phase modulated signals into a combinedsignal, generate expected signals, and correlate the combined signalwith the expected signals to form correlation signals. Such methods andsystems then store the correlation signals in a correlation signalmemory and analyze the correlation signals to extract information fromthe detected transmitted signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an implementation of theinvention and, together with the description, serve to explain theadvantages and principles of the invention. In the drawings,

FIG. 1 is a block diagram, consistent with this invention, of areceiver;

FIG. 2 is a diagram of an environment, consistent with this invention,in which the receiver of FIG. 1 may operate;

FIG. 3A is a diagram of phase modulation signals, consistent with thisinvention generated by a modulation signal generator as shown in FIG. 1;

FIG. 3B is a diagram of phase modulated signals generated by a signalmodulator as shown in FIG. 1, and a combined signal, all consistent withthis invention; and

FIG. 4 is a flow chart of a process 400 for space-division multipleaccess receiving consistent with this invention.

DETAILED DESCRIPTION OF THE INVENTION

Overview

The following description of embodiments of the present invention refersto the accompanying drawings. Where appropriate, the same referencenumbers in different drawings refer to the same or similar elements.

Methods and systems consistent with this invention overcome the hardwareand software complexity of the conventional SDMA receiver in a wirelesssystem. Such methods and systems comprise a receive element array with aplurality of receive elements. Users of the wireless system transmit aplurality of signals, each signal having a different direction orspatial location relative to the receive element array. The users maybe, for instance, mobile telephone users.

The receive element array receives the plurality of transmitted signalsin the plurality of receive elements to form a plurality of receiveelement signals. Such methods and systems form a single combined signalderived from the plurality of receive element signals, and may detecteach of the plurality of transmitted signals from the single combinedsignal based upon its different spatial location. Thus, such methods andsystems do not need multiple banks of phase shifters, multiple signalcombiners, or multiple signal detectors. Instead, such methods andsystems may detect signals from multiple users based on their differentspatial location from a single combined signal, as opposed to forming adifferent combined signal for each signal from each user and detecting asignal from each combined signal.

To achieve this, methods and systems consistent with this inventiongenerate a plurality of phase modulation signals that may be arbitraryor uncorrelated phase modulation signals, and phase modulate each of theplurality of receive element signals with a different one of the phasemodulation signals to form a plurality of phase modulated signals. Suchmethods and systems then combine the plurality of phase modulatedsignals into the combined signal, generate expected signals, andcorrelate the combined signal with the expected signals to formcorrelation signals. The expected signals are the combined signalsexpected from the directions of the users, and may be a function of thephase modulation signals and the direction of the users. Such methodsand systems then store the correlation signals in a correlation signalmemory and analyze the correlation signals to extract the transmittedinformation in the signals from the users.

Implementation Details

Methods and systems consistent with this invention receive a transmittedsignal in a receiver having a plurality of receive elements. FIG. 1 is ablock diagram of a receiver 100 consistent with this invention. Receiver100 comprises an array 1 having a plurality of receive elements, amodulation signal generator 8, a signal modulator 9, a signal combiner10, a receiver configuration memory 6, a receiver calculator 7, a signalmemory 12, a signal correlator 11, a signal router 14, a plurality ofsignal memories 15, and a signal processor 120. Receiver 100 may includeother components not specifically described above such as filters,mixers, amplifiers, and power supplies. The location of these componentsmay vary depending upon the preference of designers skilled in the art.

FIG. 2 is a diagram of an environment 200, consistent with thisinvention, in which receiver 100 may operate. In environment 200, remotetransmitter A and remote transmitter B may transmit signals 2 and 3,respectively from remote transmitter space 202. Remote transmitters Aand B may be mobile telephones, for example.

Transmitted signals 2 and 3 impinge on array 1, and the plurality ofelements receive signals 2 and 3 to form a plurality of receive elementsignals. The plurality of receive element signals are conveyed (vialines 102) to a signal modulator 9, which is described in detail below.

Methods and systems consistent with this invention generate a pluralityof phase modulation signals. Modulation signal generator 8 generatesphase modulation signals for the receive elements of array 1. Thesephase modulation signals may be arbitrary or uncorrelated (correlationless than one). The uncorrelated phase modulation signals may besubstantially uncorrelated or only slightly uncorrelated. For instance,the uncorrelated random phase signals may have a correlation less than1, but greater than 0.75; less than or equal to 0.75, but greater than0.50; less than or equal to 0.50, but greater than 0.25; less than orequal to 0.25, but greater than or equal to zero. On the other hand,some or all of these signals may be well correlated or even beidentical. The phase modulation signals may be arbitrary in that theymay not be correlated with, or otherwise dependent on, the geometry ofthe elements of array 1. The phase modulation signals may be independentof the direction of the transmitted signal.

FIG. 3A is a diagram of exemplary phase modulation signals for severalelements of array 1. As shown in FIG. 3A, modulation signal generator 8generates a phase φ₁₁ for a duration of Tc for a first phase modulationsignal 302 for a first receive element. Modulation signal generator 8then generates a phase φ₁₂ for a duration of Tc for first phasemodulation signal 302. This continues, but is shown for N periods of Tc,where Tc is the period of a “chip.” The allowed values of phase shift φfor each element of array 1 may be continuously variable from 0 to 2πradians or may be limited to a finite number of values, such as 0 and πradians. If a finite number of values for phase shift φ is used, eachelement may be assigned differing allowed values.

The same process occurs for a second phase modulation signal 304 for asecond receive element. As shown in FIG. 3A, modulation signal generator8 generates phases φ₂₁ and φ₂₂, and each for a duration of Tc, for thesecond phase modulation signal 304. This process likewise repeats for athird receive element with third phase modulation signal 306 through aJth receive element with Jth phase modulation signal 308, where J is thetotal number of receive elements in array 1. The phase modulationsignals are output to signal modulator 9. Modulation signal generator 8also outputs the phase modulation signals to receiver calculator 7,which is described in detail below. Although the phases may be random,they are known to receiver 100.

Methods and systems consistent with this invention phase modulate eachof the plurality of receive element signals with one of the phasemodulation signals to form a plurality of phase modulated signals. Thus,signal modulator 9 phase modulates, or “chips,” each element signal withone of the phase modulation signals generated by modulation signalgenerator 8. FIG. 3B is a diagram of phase modulated signals consistentwith this invention. As shown in FIG. 3B, a first chip of first receiveelement phase modulated signal 310 is equal to the first receive elementsignal, but phase shifted by φ₁₁, the first phase of phase modulationsignal 302. Likewise, a second chip of phase modulated signal 310 isequal to the first receive element signal, but phase shifted by φ₁₂, thesecond phase of phase modulation signal 302. Likewise, the secondthrough the Jth receive element signals are phase modulated to formsecond 312 through Jth 316 phase modulated signals.

Phase modulated signals 310-316 output from signal modulator 9 to signalcombiner 10 (via lines 104). Methods and systems consistent with thisinvention combine the plurality of phase modulated signals into acombined signal 318. Thus, signal combiner 10 combines the phasemodulated signals into combined signal 318. In one embodiment, signalcombiner 10 sums, chip by chip, the plurality of phase modulated signalsto form combined signal 318. For example, all of the first chips fromfirst phase modulated signal 310 through Jth phase modulated signal 316are added to form a combined signal first chip 320, all of the secondchips from phase modulated signal 310 through Jth phase modulated signal316 are added to form a combined second chip 322, and so forth. Eachchip of combined signal 318 may have a vector magnitude that conforms toa Rayleigh density function and may have a random phase angle. Combinedsignal 318 is output from signal combiner 10 to a signal correlator 11(via line 106).

Methods and systems consistent with this invention generate an expectedsignal. The expected signal is the signal that the combined signal 318is expected to be if an unmodulated carrier were transmitted from aparticular direction relative to array 1. Receiver calculator 7calculates the expected signal. For example, referring to FIG. 2,receive calculator 7 may generate an expected signal for a carrier fromthe direction of transmitter A. Receiver calculator 7 inputs informationfrom modulation signal generator 8 and receiver configuration memory 6in order to calculate the expected signal. Receiver configuration memory6 may provide information that affects the amplitude, phase, andpolarization of receive element signals and phase modulated signalsbefore being combined in signal combiner 10. This information mayinclude the carrier frequency of transmitted signals 2 and 3, theirestimated direction, the configuration of the receive elements withinarray 1, and the transmission line lengths of the elements. Modulationsignal generator 8 may provide information giving the relative phase ofeach chip within the phase modulation signals 302-308. Receivercalculator 7 calculates and outputs the expected signal to the signalmemory 12 for temporary storage. The expected signal is output from thesignal memory 12 and input to signal correlator 11. Because thepolarization of the transmitted signal may influence the phase andmagnitude of the combined signal, receiver calculator 7 may calculatethe expected signal based upon an assumed polarization of thetransmitted signal.

Methods and systems consistent with this invention cross-correlatecombined signal 318 with the expected signal to form a correlationsignal. Signal correlator 11 inputs combined signal 318 and the expectedsignal and correlates the two signals. In one embodiment, signalcorrelator 11 may cross-correlate the corresponding N consecutive chipsof combined signal 318 and the expected signal. In this embodiment, thevalue N may be 50. Signal correlator 11 may perform a newcross-correlation between combined signal 318 and the expected signaleach time N new chips (or time period N×Tc) of combined signal 318enters correlator 11. Each time a new correlation is performed, receivercalculator 7 may update the expected signal to include the next N chipsand may delete the previous chips so that the value of N may remain 50,for example. Signal correlator 11 produces an output that is a measureof the cross-correlation of combined signal 318 and the expected signal.In the example of FIG. 2, signal correlator 11 produces an output thatis the correlation signal for receiver 100 “looking” in the direction ofremote transmitter A provided that the expected signal beingcross-correlated with combined signal 318 is that from the direction oftransmitter A. The correlation signal is output to signal router 14.

Methods and systems consistent with this invention may generate aplurality of expected signals from a plurality of directions and maycorrelate combined signal 318 with the plurality of expected signals toform a plurality of correlation signals. For example, referring to FIG.2, receive calculator may generate an expected signal for a carrier fromthe direction of transmitter A and an expected signal for a carrier ofthe same or different frequency from the direction of transmitter B.Thus, receiver 100 may simultaneously “look” in multiple (M) directionsat one time, and receiver calculator 7 may generate M expected signalsand signal correlator 11 may cross-correlate M expected signals withcombined signal 318 to form M correlation signals. Each correlationsignal is the detection signal for receiver 100 “looking” in that oneparticular direction. The M correlation signals are output to signalrouter 14 (via line 108).

Methods and systems consistent with this invention store the Mcorrelation signals in correlation signal memory 15 and analyze thecorrelation signals. Using signal processor 120, information such asvoice or other data is extracted from the correlation signals. Signalrouter 14 passes each of the M correlation signals to one of the severalsignal memory units 1 to M, respectively. Signal memory units 1 to Mstore successive correlation signals from an assigned direction 1 to M,respectively.

If the processing is at sufficiently high speeds, receiver 100 cansimultaneously process and detect signals from many directions.Alternatively, signal memories 1 to M store correlation signals fordifferent individual transmitters, such as transmitter A or B. This isuseful if a transmitter is mobile, and thus constantly changingdirection with respect to receiver 100. In this case, the direction usedby receiver calculator 7 to establish the expected signal for a mobiletransmitter is continuously updated to correspond to the currenttransmitter location.

Array 1 may not have directional characteristics, but rather it may beisotropic (omnidirectional). The arbitrary relationship of the phasemodulation signals may give a combined signal block of N chips,regardless of the transmitted signal's direction of arrival, the sameaverage energy within the receiver. Array 1 and receiver 100 also may bedesigned such that it is isotropic with respect to transmitted signalswithin a more limited transmitter space.

FIG. 4 is a flow chart of a process 400 for space-division multipleaccess receiving consistent with this invention. First, methods andsystems, consistent with this invention receive a transmitted signal inthe plurality of receive elements to form a plurality of receive elementsignals (step 402). Such systems then generate a plurality of phasemodulation signals (step 404) and phase modulate each of the pluralityof receive element signals with a different one of the phase modulationsignals to form a plurality of phase modulated signals (step 406). Suchmethods and systems then combine the plurality of phase modulatedsignals into a combined signal (step 408). Such methods and systems thengenerate an expected signal (step 410) and cross-correlate combinedsignal 318 with the expected signal to form a correlation signal (step412). Such methods and systems then store the correlation signal in acorrelation, signal memory and analyze, the correlation signal (step414).

Expected Signal Polarization

The polarization of the transmitted signal, in general, may affect theexpected signal. If the polarization of the transmitted signal is knownin advance, then receiver calculator 7 may use this value in calculatingthe expected signal. If the value of the polarization is not known inadvance, receiver calculator 7 has several options. One option is toassume a value for the polarization and calculate the expected signalbased upon this assumed value. In this option, the component of thepolarization of the transmitted signal that coincides with the assumedpolarization is detected.

Another option is for receiver calculator 7 to calculate two expectedsignals. The first expected signal is calculated based upon an assumedpolarization as before, and the second expected signal is calculatedbased upon a polarization normal (orthogonal) to the first polarization.The transmitted signal is detected by separately correlating thecombined signal with the first and second expected signals, forming twocorrelation signals. These two correlation signals may be processedindividually or may be combined by signal processor 120 in order toextract the information from the transmitted signal.

Yet another option is to calculate two expected signals as before, thefirst expected signal based upon an assumed polarization and the secondexpected signal based upon a normal (orthogonal) polarization. In thisoption, the two expected signals are summed or otherwise combined toform a third expected signal. The transmitted signal is detected bycorrelating the combined signal with the third expected signal.Regardless of the polarization of the transmitted signal, in this optionthere is good correlation with the third expected signal.

These techniques, along with others, devised by those skilled in the artmay be used to detect and extract information from transmitted signalswith any type of polarization characteristics, such as linear, circular,or elliptical.

Processing Gain

Methods and systems consistent with this invention may generate aplurality of phase modulation signals, wherein the phase modulationsignals have a chipping rate and the chipping rate exceeds a modulationrate of the transmitted signal. In one embodiment, signal modulator 9may chip the received element signals continuously at a rate that is atleast one-hundred times the period of the highest modulation frequencyof the transmitted signal. This chipping rate may allow signalcorrelator 11, which in one embodiment processes a block of fifty chipsat one time, to contain no more than one half-cycle or one half-wave ofthe modulation signal impressed upon any carrier, thus meeting theminimum Nyquist sampling rate. Thus, in one embodiment, the correlationlength of fifty chips at a chipping rate of at least one-hundred timesthe highest modulation rate corresponds to the maximum Nyquist samplinginterval. This may permit complete recovery of the modulationinformation from any carrier. Values of N other than 50 are possible,and satisfying the minimum Nyquist rate may result in different chippingrates.

The amplitude and phase of each chip within combined signal 318 (FIG.3B) on line 106 (FIG. 1) may depend upon the angle of arrival of atransmitted signal at the receive elements of array 1. Receivercalculator 7, in order to differentiate between signals arriving fromdifferent directions, anticipates and calculates for each direction theexpected chip amplitude and phase patterns that may be present withincombined signal 318. For each direction, signal correlator 11cross-correlates the expected chip patterns i.e., the expected signal,with combined signal 318. In signal memory 12, there are K expected chippatterns from K different directions. In one embodiment, K is equal toM, as discussed above.

Signal correlator 11 may have a processing gain of √{square root over(N)}, where N is the number of chips, within the combined signal 318,processed together at the same time. The N chips form a block ofduration T. The cross-correlation described is between combined signal318 and the plurality of K expected signals.

In one embodiment, the value for processing gain is established asfollows. A combined signal block containing N chips (spanning the timeinterval from a start time t1 of first chip to an end time t1+T of theNth chip) has a correlation energy expression ofR _(ER)(K,θ)=∫_(t1) ^(t1+T){{right arrow over (v_(EK))}(t)}·{{rightarrow over (v_(R))}(t)e ^(+jθ) }dt,where {right arrow over (v_(R))}(t) is combined signal 318 comprising Nchips and {right arrow over (v_(ER))}(t) is the corresponding Kthexpected signal also comprising N chips. Each clip of {right arrow over(v_(R))}(t) and {right arrow over (v_(EK))}(t) has a square value ofα_(R) ² and α_(EK) ² respectively and a root-mean-square (ms) value ofα_(R) and α_(EK) respectively. They may be substantially random vectorsthat conform to Rayleigh density functions with random phase andexpected magnitude values of

$\frac{\sqrt{\pi}}{2}\alpha_{R}\mspace{14mu}{and}\mspace{14mu}\frac{\sqrt{\pi}}{2}\alpha_{EK}$respectively. These substantially random vectors may each be composed ofthe sum of random phase chips within the phase modulated signals inputto signal combiner 10. The phase shift term e^(+jθ) may be appliedequally to all chips of a combined signal block where the parameter θmay be chosen to maximize the correlation output for each processedcombined signal block. In wireless systems where the transmitted signalis phase modulated, as with QPSK, the parameter θ is part of thecorrelation signal and may be used to derive the carrier phaseinformation. In systems where the transmitted signal is amplitudemodulated, the magnitude of the cross-correlation is part of thecorrelation signal and may be used to derive the carrier amplitudeinformation.

The magnitude of the correlation energy of N chips that are wellcorrelated is

${N\;\alpha_{R}{\alpha_{EK}\left( \frac{T}{N} \right)}},$where T is the combined signal block duration and where

$\left( \frac{T}{N} \right)$is the time interval of a single chip, or Tc.

If, on the other hand, the combined signal block of chips is random withrespect to the corresponding expected signal block of chips, i.e., theyare not correlated, the magnitude of the correlation energy of the Nchips is

$\sqrt{N\;\alpha_{R}}{{\alpha_{EK}\left( \frac{T}{N} \right)}.}$In this case, the N combined signal vectors (chips) have random phaseswith respect to the corresponding N expected signal vectors (chips). Thesum of N random vectors (with r.m.s. value of α_(R)) is two-dimensionalGaussian (with r.m.s. value of √{square root over (Nα_(R))}). Thistwo-dimensional Gaussian density function may also be described as aRayleigh density function.

The value for processing gain is found by forming the ratio of thecorrelator output for a well correlated signal

$\sqrt{N\;\alpha_{R}}{\alpha_{EK}\left( \frac{T}{N} \right)}$and an uncorrelated signal

$\sqrt{N\;\alpha_{R}}{{\alpha_{EK}\left( \frac{T}{N} \right)}.}$

One skilled in the art appreciates that numerous variations to thissystem exist. For example, methods and systems consistent with thisinvention also may function with acoustic signals, not onlyelectromagnetic signals. For instance, the transmitted signals may beacoustic signals conveyed through water, and the receive elementsignals, the combined signal, and the expected signals may all representacoustic signals in a receiver and processor. Such a system may providefor an undersea data link or any type of sonic signal detection. In sucha system, the receive elements of array 1 are acoustic sensors.

Also, it is generally easier for signal processors to generatepseudo-random numbers rather than purely random numbers, and thus theterm “random” includes “pseudo-random.” Therefore, modulation signalgenerator 8 may generate pseudo-random phase modulation signals andsignal modulator 9 may generate pseudo-random phase modulated signals.This applies for phase modulation signals φ that are either continuouslyvariable or limited to a finite number of values.

Further, the technique used for comparing the combined signal with theexpected signals, herein described as correlation, may draw upon anysuitable signal comparison techniques well known in signal processingfor recovering the magnitude and phase information from the transmittedsignal.

Further still, array 1 may take on many different shapes. For example,array 1 may be flat, spherical, or cylindrical. It may also conform to asurface, such as the outside of an airplane or an automobile.

Lastly, all or some of the functions performed by signal modulator 9,signal combiner 10, modulation signal generator 8, signal memory 12,signal correlator 11, signal router 14, receiver calculator 7, receiverconfiguration memory 6, and signal processor 120 may be implemented insoftware, not necessarily hardware.

Although methods and systems consistent with the present invention havebeen described with reference to an embodiment thereof, those skilled inthe art know various changes in form and detail that may be made withoutdeparting from the spirit and scope of the present invention as definedin the appended claims and their full scope of equivalents.

The invention claimed is:
 1. A method comprising: receiving a pluralityof electromagnetic radiation signals at an array; generating a pluralityof random modulation signals; modulating each of the plurality ofreceived signals with a respective one of the random modulation signalsto form a plurality of modulated signals; summing the plurality ofmodulated signals into a combined signal; and correlating the combinedsignal with an expected signal to form a correlation signal.
 2. Themethod of claim 1, further comprising generating the expected signal. 3.The method of claim 2, wherein receiving the plurality ofelectromagnetic radiation signals includes receiving the plurality ofelectromagnetic signals over a period of time, the method furthercomprising: generating successive pluralities of random modulationsignals; and modulating the plurality of received signals with thesuccessive pluralities of random modulation signals to form successivepluralities of modulated signals corresponding to the period of time. 4.The method of claim 3, wherein summing the plurality of modulatedsignals includes combining the successive pluralities of modulatedsignals to form a sequence of combined signals corresponding to theperiod of time.
 5. The method of claim 4, wherein each of the successivepluralities of random modulation signals is uncorrelated.
 6. The methodof claim 5, wherein correlating includes correlating the sequence ofcombined signals with the expected signal to form a correlation signal.7. The method of claim 3, wherein generating an expected signalcomprises generating the expected signal as a function of the successivepluralities of random modulation signals.
 8. The method of claim 1,wherein the array includes a plurality of elements and each elementreceives one of the plurality of electromagnetic radiation signals.
 9. Amethod comprising: receiving a plurality of electromagnetic radiationsignals at an array; generating a plurality of modulation signals, theplurality of modulation signals being uncorrelated; modulating each ofthe plurality of received signals with a respective one of the pluralityof modulation signals to form a plurality of modulated signals; summingthe plurality of modulated signals into a combined signal; andcorrelating the combined signal with an expected signal to form acorrelation signal.
 10. The method of claim 9, wherein receiving theplurality f electromagnetic signals includes: receiving the plurality ofelectromagnetic radiation signals over a period of time, generatingsuccessive pluralities of modulation signals, each of the plurality ofmodulation signals being uncorrelated; and modulating the plurality ofreceived signals with the successive pluralities of modulation signalsto form successive pluralities of modulated signals corresponding to theperiod of time.
 11. The method of claim 10, further comprisinggenerating an expected signal.
 12. The method of claim 11, whereinsumming the plurality of modulated signals into a combined signalincludes combining the successive pluralities of modulated signals toform a sequence of combined signals corresponding to the period of time.13. The method of claim 12, wherein the successive pluralities ofmodulation signals include successive pluralities of random modulationsignals.
 14. The method of claim 13, wherein generating an expectedsignal comprises generating the expected signal as a function of thesuccessive pluralities of modulation signals.
 15. The method of claim14, wherein correlating the combined signal with the expected signalincludes correlating the sequence of combined signals with the expectedsignal.
 16. An apparatus comprising: an array to receive a plurality ofelectromagnetic radiation signals; a modulator to modulate each of theplurality of received signals with a respective one of a plurality ofrandom modulation signals to form a plurality of modulated signals; anda signal combiner to combine the plurality of modulated signals into acombined signal, wherein the array is configured to receive theplurality of electromagnetic signals over a period of time, wherein themodulator is configured to modulate the plurality of received signalswith successive pluralities of random modulation signals to formsuccessive pluralities of modulated signals corresponding to the periodof time.
 17. The apparatus of claim 16, wherein the signal combiner isconfigured to combine the pluralities of modulated signals to form asequence of combined signals corresponding to the period of time. 18.The apparatus of claim 17, wherein each of the successive pluralities ofrandom modulation signals are uncorrelated.
 19. The apparatus of claim17, further comprising a correlator to correlate the sequence ofcombined signals with an expected signal to form a correlation signal.